1. Field of the Invention
This invention relates to polyethylene composition having excellent melting characteristics and moldability and giving molded articles with good surface properties. More particularly, it relates to polyethylene composition which is formed by blending polyethylene obtained according to a two-step polymerization process using a Ziegler catalyst, with high-molecular-weight polyethylene obtained with the aid of a catalyst containing a chromium compound, which has excellent environmental stress cracking resistance (hereinafter referred to as ESCR) and gives molded articles with good surface properties, and which is suitable for use in blow molding and extrusion molding.
2. Description of the Prior Art
Generally, polyolefin used for the fabrication of molded articles such as bottles, cable conduits, very thin films and the like must withstand molding conditions satisfactorily in the plastic state so as to be easily formed into the desired shape. To this end, it is desirable to use polyolefin having a high melt index (i.e., having a low average molecular weight). However, such high-melt-index polyolefin inevitably have low strength characteristics such as impact strength, tensile strength and the like. On the other hand, low-melt-index polyolefin have high strength, but suffer from poor moldability. It is known that this problem can be solved by using polyolefin having a wide molecular-weight distribution.
In recent years, the properties required for polyethylene have become more and more diverse. Moreover, from the viewpoint of saving of resources, there is a growing tendency to minimize the amount of resin used without impairing its properties. In the case of bottles and films, for example, the recent tendency of the art is to minimize their thickness while retaining their strength. Accordingly, there is a strong demand for polyethylene which, when used in small amounts, can exhibit good moldability and excellent properties such as impact strength, tensile strength, ESCR, etc.
Furthermore, polyethylene has recently attracted much attention as a material for the fabrication of vessels of very large size, such as automobile fuel tanks, drums and the like. The polyethylene used in such applications must have high melt tension, high impact strength and good ESCR. As described above, these requirements can be satisfied by widening its molecular-weight distribution.
There have been proposed a number of processes for the preparation of polyethylene having a wide molecular-weight distribution.
One type of processes comprise blending high-molecular-weight polyethylene with low-molecular-weight polyethylene (Japanese Patent Publication No. 3215/'70 and Japanese Patent Laid-Open No. 133136/'82).
Another type of processes involves multistep polymerization including two or more steps (Japanese Patent Publication No. 11349/'71 and Japanese Patent Laid-Open No. 19788/'77).
Although the polymer obtained by these processes have a wide molecular-weight distribution and fairly good ESCR, they still have the disadvantage of exhibiting inadequate melt viscoelasticity at the time of molding and tending to give molded articles with gel particles and/or shark skin.
Japanese Patent Laid-Open No. 196345/'84 has proposed polyethylene composition having improved moldability and physical properties. It is described therein that "improvements in moldability and physical properties have been achieved by connecting the low-molecular-weight and high-molecular-weight fractions of polyethylene prepared by two-step polymerization using a magnesium-containing Ziegler catalyst, with polyethylene prepared with the aid of a catalyst comprising a combination of a chromium compound and an organometallic compound and characterized by an intermediate molecular weight, a relatively wide molecular-weight distribution and a moderate Barus effect, so that the molecules are properly intertwined to assume the optimum molecular structure."
Moreover, Japanese Patent Laid-Open No. 100444/'79 has provided "polyethylene resin composition having improved ESCR which is formed by blending homopolymer of ethylene, or copolymer of ethylene and an .alpha.-olefin, having a relatively low molecular weight and high density of not less than 0.955 g/cm.sup.3, with a copolymer of ethylene and an .alpha.-olefin of 4 or more carbon atoms having a relatively high molecluar weight and a low density 0.910 to 0.950 g/cm.sup.3. "
Although the compositions provided by the aforementioned Japanese Patent Laid-Open Nos. 196345/'84 and 100444/'79 have improved moldability and ESCR, their melt tension cannot be regarded as adequate and still remains to be improved. Moreover, since polymer having substantially different molecular weights are blended, they do not have satisfactorily good dispersibility (or compatibility) and the resulting composition tends to give molded articles with shark skin. In order to improve the compatibility, it is conceivable to raise the blending temperature or use a mixer having high shearing force. However, this may undesirably cause breakage or deterioration of the polymer chains.